a. Field of the Invention
This invention relates to a device for feeding liquid. Generally, the technique of feeding liquid has been widely used in many practical fields such as, for example, feeding devices for printing ink, liquid applying devices utilizing a coater, devices for maintaining constant a concentration of developing liquid, feeding devices for offset preventing liquid to an image fixing roller for electrophotographic apparatus, and others. In this liquid feeding technique, however, maintenance of the liquid feeding quantity at a constant level is one of the important problems. In order to attain the constant feeding quantity, there have been made various efforts, and the present invention also aims at such solution to the problem.
b. Description of Prior Art
For the purpose of explanations of the prior known art to more readily understand the unique features of the present invention, an offset preventing liquid feeding device for a heat-roller image-fixing device which has already been put into practice in the image fixing section of the electrophotographic apparatus is taken as an example. It should, however, be understood that the present invention as will be described hereinafter is not limited to this particular device alone.
In the electrophotographic method, there have been practised the process steps of forming an electrostatic latent image in utilization of a photoconductive substance, developing the latent image with charged fine powder, then transferring the developed image onto an image holding member, and finally fixing the fine powder onto the image holding member by heat. When the dry powder development system is adopted, there is widely used a heat-roller image-fixing device, in which the image holding member carrying thereon the developed image is caused to pass between a heated roller and a pressure roller. In this kind of image-fixing device, there tends to occur adherence of the fine powder constituting the developed image to the heating roller due to its fusion, which has been one of the causes of deterioration in the image quality. In order to prevent this so-called "offset" phenomenon, there has been practised a method, in which silicone rubber, tetrafluoroethylene, and like other substances having a small surface free energy is used as a material for the surface layer of the heating roller to contact with the developed image, and a film of the offset preventing liquid such as silicone oil, etc. is formed on the surface layer. While the offset phenomenon can be avoided by this method, there would arise a new problem depending on the quantity of the offset preventing liquid. That is, when the feeding quantity of the offset preventing liquid is small, the offset phenomenon tends to take place readily, while, when the quantity is large, excessive amount of the liquid brings about swelling of the silicone rubber and like material to lower its durability, and produces stain on the developed image holding member, and also replenishment of the offset preventing liquid becomes necessary to be done in a shorter period of time.
Also, in the conventional offset preventing liquid feeding device for the heat-roll image-fixing device, the liquid feeding quantity fluctuates between the commencement of the feeding operation and the long hours' continuous feeding operation. Particularly, the feeding quantity at the commencement of the operation is excessive in comparison with that during the long hours' continuous operation, and the abovementioned inconvenience occurs accordingly. In such device, since the liquid feeding quantity is so determined that the offset phenomenon may be avoided even when the feeding quantity is at the minimum, there is no possibility at all to solve the abovementioned inconvenience which is brought about by increase in the feeding quantity over its appropriate one.